The present invention relates generally to an antilock brake control system for vehicles and more particularly, to the antilock brake control system which is responsive to braking upon cruising at high speed or on snow.
Conventionally, with the antilock brake control system which prevents lock of wheels upon sudden braking of a vehicle such as motor vehicle, a pseudo vehicular body speed is determined in accordance with a detected wheel speed so as to establish a target wheel speed which is in a predetermined ratio with respect to the pseudo vehicular body speed. When the detected wheel speed is smaller than the target wheel speed, pressure decreasing control is carried out which starts a reduction in brake fluid pressure within wheel cylinders. See, for example, JP-A 60-261767.
With such known antilock brake control system, however, since the ratio of the target wheel speed to the pseudo vehicular body speed which is relevant to start of pressure decreasing control is established to be constant regardless of the magnitude of a vehicular speed, i.e., a skid ratio is established to be always constant, there arises a problem that in braking operation throughout the entire area of a cruising speed of the vehicle, stable behavior of the vehicle may not be always obtained. By way of example, if the skid ratio is established at a constant value so as to conform to braking of the vehicle upon relatively low speed, a skid amount of wheels is increased in braking of the vehicle upon high speed, deteriorating stability of the vehicle.
The cause thereof is as follows: Since a friction coefficient .mu. between the wheels and the road (designated "road .mu.") is decreased with an increase in vehicular speed, a brake fluid pressure corresponding to a peak value of the road .mu. (designated "lock fluid pressure" since the wheels tend toward lock when exceeding this) is also decreased. With the lock fluid pressure decreased, an overshoot part of the brake fluid pressure to the lock fluid pressure is increased, enlarging the skid amount of the wheels. Referring to FIG. 11, P.sub.W designates a brake fluid pressure applied to the wheel cylinders, which is increased at a point "a" where braking starts. V designates a pseudo vehicular body speed, V.sub.W designates a wheel speed, V.sub.S1 designates a target wheel speed, and S.sub.r1 designates a target skid ratio of the wheel speed to the pseudo vehicular body speed. It is to be noted that fully-drawn lines indicate a case that the target skid ratio S.sub.r1 is established at a constant value upon high speed as well as upon low speed.
Thus, the wheel speed V.sub.W is decreased with an increase in brake fluid pressure P.sub.W. However, since the target wheel speed V.sub.S1 at which a pressure reduction starts is established in a constant ratio with respect to the pseudo vehicular body speed V, the target wheel speed V.sub.S1 is large upon high speed. Therefore, a pressure increase can be continued even if the brake fluid pressure P.sub.W exceeds a lock fluid pressure P.sub.L, producing an overshoot part A of the brake fluid pressure between points "b" and "c". At the point "c" at which the wheel speed V.sub.W reaches the target wheel speed V.sub.S1, pressure decreasing control starts which induces the greater skid amount C of the wheels due to the overshoot part A.
On the other hand, recovery of the wheel speed V.sub.W is expected according to pressure decreasing control, which is delayed, however, due to the greater skid amount C of the wheels and inertia thereof, producing an excessive reduction B in the brake fluid pressure P.sub.W. This excessive pressure reduction may cause a long-sustained short braking force state (a point "d" to a point "e").
It is, therefore, an object of the present invention to provide an antilock brake control system which ensures stable behavior of a vehicle with regard to braking throughout the entire area of a cruising speed of the vehicle.